Damage caused by moisture in monumental brickwork of the Upper Castle in Opole

Jan Kubik, Zbigniew Perkowski, Andrzej Marynowicz

Opole University of Technology

Abstract

In the paper one presented studies concerning renovation of the monumental building with strongly moist brickwork. In order to determine moisture contents in the masonry one used a quick and efficient method, which consisted in measurements of relative air humidity in the special openings drilled in the wall and next, on that basis, in estimation of humidity of material with use of the proper sorption isotherms for brick. The presented approach can be used easily in a building engineering practice.

The investigations presented in the work concern an evaluation of damage caused by actions of moisture flows in the monumental masonry belonging to the remainder of the Upper Castle in Opole (Fig.1) - two towers and the retaining wall. This assessment was finally a basis for the design of repair works which were started on the Castle`s brickwalls in 2006 (Fig.1). A short review of these works is also presented in the end of the paper.

a) b)

Figure1. View of the remainder from the Upper Castle in Opole
(two towers and retaining wall):a) before b) just after the
preliminary repair works.

At present, the building is administrated by the Complex of Mechanical Schools in Opole. Interior of the bigger tower is all the time used by the administration of the school. However, history of the Opole defensive walls goes back to the 10th century [6]. In the beginning they were made from wood and ground and the first brickwalls were built in the 13th century. Period of the greatest development occurred in the 16th century. Starting from the 17th century because of numerous wars made in Silesia, development of artillery, the town walls of Opole started to deteriorate and lost their strategic sense. Finally, in the 19th century the authorities of that time ordered to demolish them. That was why, fragments of the defensive walls saved till now were reconstructed in any way during the 19th century and integrated with younger buildings built at that time. These actions often implicated their bad technical condition. In face of progressing deterioration of the rests of the monument and their possible loss, the present town-authorities decided on their renovation also with participation of scientific workers. This action was a basis of this elaboration.

2. Evaluation of damage in the masonry

After the visual inspection it was found that the Castle's brickwalls were strongly moist, especially in the wall copings and in the lower parts touching the ground - mainly marls. Evaluation of water-and-ground conditions excluded that water-table reached the level of the building's foundations [3], so in the face of luck of any damp insulation under the ground level, one found that capillary suction of rainfall water was the main reason of the problem.

The walls in the lower parts are also very thick - about 3m - and consist of three parts: two external ones made from brick with lime mortar joints and the internal one made from limestone. One could also find many repairs in the walls made from 19th and 20th century brick with cement mortar joints. In the consequence of lack of any antimoisture protection by many years one could observe damage like: spalling, holes, bulbs, strong destruction of the wall copings and small amount of cracks which were caused by moisture and other climatic impacts (Fig. 2,3).

a)
b)

c)

Figure 2. Examples of damage in the masonry:
a) damp and spalling on the bottom face of the retaining wall,
b) bulb in the wall of the smaller tower,
c) destruction of the wall coping.

Figure 3. Making inventory of damage on the front
of the Castle. Location of openings for moisture measurements.

Fortunately, one did not found slips or cracks in joints of the masonry testifying that the building could settle non-uniformly. That circumstance allowed the authors to restrict the actions related with making of the renovation design to evaluation of moisture contents in the masonry (point 3) and proposing on that basis new damp insulation and repairs of the wall face (point 4).

3. Measurements of moisture contents in the masonry

Measurements of moisture contents in the masonry were made in parallel in 9 points - special openings 20cm deep and 0,8cm wide protected from weather conditions by the rubber stoppers. Their location is shown on the Fig. 3. The investigations of moisture contents had to be conducted in a non-invasive way because of the monumental character of the building. That was why, evaluation of moisture contents was made only indirectly by measurements of relative air humidity in the openings. Measurements were made in June - 10 times at regular 3 days intervals with use of the electronic meter Almemo. It was found that mean relative air humidity in the holes was on the high level 88-98% (Table 2), while mean relative air humidity in the surrounding of the building was equal to 52% and mean temperature to 19oC. Moreover, air humidity in the openings was on the constant level regardless of weather summer conditions. In the opinion of the authors, that was related with luck of any antimoisture protection by many years and big mass of the wall causing considerable "moisture inertia" of the building. The results showed that the highest humidity of the masonry had been permanently over the hygroscopic range.

In order to estimate humidity of the masonry in the surrounding of the openings, first one determined maximal saturation in normal conditions for small probes of brick used in the monumental masonry - medieval one and one coming from the 19th century reconstruction. Results are shown in Table 1 as a mean of six probes. Next one made sorption isotherms for contemporary brick of average compressive strength 15MPa and 30MPa comparing humidity of the probes in different stationary hygro-thermal conditions (T=20oC) with relative air humidity in the drilled openings of the identical diameter as those ones in the masonry (Fig. 4). These sorption isotherms were obtained as an averaging result from 6 probes according to the relation [2,4]

The mentioned types of brick had maximal saturation close to the old ones (Table 1). So results for the brick of strength 15MPa were attributed to the older one and the brick of strength 30 MPa to the younger one. The brick of the worse quality was able to store about 5-10 times more moisture in comparison with the second type of brick in the same conditions. On the basis of these investigations one found that the medieval older brick in the surrounding of the openings had humidity within the range of 3-10,6 percentages by weight and the 19th century brick within the range of 0,6-1,7 percentage by weight (Table 2). It should be mentioned that the considerations presented above are only an estimation because connections between relative air humidity in the pores of material and its total humidity are not unique for high moisture contents. It is caused among other things by occurrence of hysteresis loop in sorption process and by the fact that considerable changes of total moisture contents in material accompany small changes of relative air humidity in its pores over the hygroscopic range (for example [1,5]). However, it could be found that total humidity of the building was permanently on the high level often over the hygroscopic range regardless of the weather conditions so, in order to better the technical state and the general looks of the masonry, the humidity level had to be reduced.

Table 1. Mean maximal water contents in the brick probes used for the tests under the normal conditions. Mean relative air humidity in the openings drilled in the probes of contemporary brick during maximal saturation.

Material

Probes of the older brick

Probes of the younger brick

Probes of the contemporary brickfc,mean=15[MPa]

Probes of the contemporary brickfc,mean=30[MPa]

Mean maximal water contents [% by weight]

14,3

3,4

18,5

1,5

Mean relative air humidity in the opening [%]

-

-

99,3

99,9

Table 2. Mean relative air humidity in
the openings drilled in the masonry and
water contents estimated in their surrounding.

No. of the opening

1

2

3

4

5

6

7

8

9

Mean relative air humidity [%]

98

96

93

87

88

90

95

97

93

Estimated water contents[% by weight]

The older brick

10,6

6,7

4,2

2,3

2,5

3,0

5,6

8,2

4,2

The younger brick

1,7

1,4

1,0

0,6

0,7

0,8

1,2

1,5

1,0

4. Renovation

All proposed actions concerning betterment of the building's technical state were concentrated on conversion the monument into so called "stable ruin" because of economic aspects. Creation of such a state would save the masonry in proper condition as a tourist-attraction with simultaneous minimisation of living costs borne by the town-authorities.

The plan of repair works consisted of the following actions:

cleaning of the face of the masonry from dust,
grime and small plants

making of bowstrings in the smaller town
strengthening the bulb shown on the Fig. 2b

refilling and reconstruction of
destroyed fragments of the masonry (cracks, holes,
the wall copings etc.) with preservation of the original
colours and bond

protection of the masonry under the
ground level by insulating membranes enabling the masonry
"breathing" under the ground level

renovation of gutter and drainage
system

surface hydrophobisation of the wall
copings

making of concrete shields 3m wide
on the ground at the masonry protecting the ground from
excessive penetration of rainfall

drilling of Knappen's holes of 40mm
diameter and 2m deep in regular 0,5m intervals in the bottom
part of the masonry

Making of Knappens's holes was motivated
by impossibility of drying of the wall in the bottom part
with use of modern techniques - for example microwave one
because of the big thickness of the wall. Also the considerable
age of the structure was a premise in order to avoid rapid drying,
which could cause cracks in consequence of large distortion shrinkage
and thermal stresses. Moreover, after reduction of relative air humidity
to 70% in pores of the wall, the holes will be used to application of
injection preparations in order to make volumetric damp insulation
in the bottom of the masonry. Because of the big thickness of the
wall, the natural drying process may last 2-3 years and it will
be subjected to monitoring.

Conclusions

In the paper one presented studies concerning the renovation of the monumental building with strongly moist brickwork. One used a method of quick and simple estimation of the humidity level in the masonry. The method was based on the comparison of relative air humidity in the special openings drilled in brick with its total humidity on the basis of sorption isotherms. The presented approach can be used successfully in a building engineering practice without any difficulties. In the case discussed in this paper the possibility of simple determination of water contents in the masonry was important for planning of the further repair works which were oriented on transformation of the building into one of tourist-attractions in Opole.